It is well known that ethylenically unsaturated monomers like vinyl aromatic compounds, such as styrene, .alpha.-methylstyrene, vinyltoluene or divinylbenzene or acrylic monomers, such as acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid and their esters and amides, or unsaturated esters such as vinyl acetate or unsaturated polyesters have a strong tendency to polymerize when subjected to elevated temperatures. Manufacturing processes for such monomers typically include distillations or handling at elevated temperatures.
Known inhibitors of acrylic monomer polymerization include phenothiazine, hydroquinone monomethyl ether, methylene blue and nitroxides. Phenothiazine, while unable to totally inhibit polymerization of acrylic monomers, is a commonly used co-additive. Recent patents claim phenylenediamines with soluble transition metal salts (U.S. Pat. No. 5,221,764), and aryl N-nitroso compounds (EP 0 522 709 A2) are active in acrylic monomer stabilization. U.S. Patent No. 5,504,243 teaches a ternary mixture of a soluble transition metal salt, nitroso compound and nitroxide are also effective as monomer inhibitors. However, there still remains a need for a compound to improve the stability of acrylic monomers during their processing. The need exists for a stable polymerization inhibitor system which will effectively and safely prevent the premature polymerization of unsaturated acrylate monomers during distillation and other purification processes, particularly if air is absent, and during organic-aqueous phase separations.
As indicated above, it is known that acrylonitrile, acrylic acid, methacrylic acid and their respective esters, which are generically referred to in this application as acrylates, have a tendency to undergo unwanted and premature polymerization at elevated temperatures. The industrial production of acrylates also yields several byproducts from which the desired monomer must be separated. One stage of this purification usually involves the partition of the acrylate between an organic and an aqueous phase. During this purification and subsequent steps, the acrylate can undergo a thermally induced polymerization. This undesired reaction must be limited and hopefully entirely repressed to insure that the reactors, tanks, pipes, etc. used to make, store and transport the monomer remain free or essentially free of high molecular weight polymeric material.
The production of acylic acid usually involves the catalytic gas-phase oxidation of propylene. During the purification processing of the monomer, the reaction stream often undergoes a partition between an organic and an aqueous phase. During this partition, a particular polymerization inhibitor may not partition sufficiently well into each of the two phases where it is needed to prevent the monomer from undergoing premature and unwanted polymerization. The instant invention pertains to the use of a mixture of two or more nitroxides which when used together provide synergistic stabilization efficacy to the acylate monomer during its preparation and purification, especially when it undergoes partition between an organic medium and water or even in a solely organic system.